The present invention relates to photomultiplier tubes and particularly to an improved secondary emission surface for a primary dynode.
Electron emissive electrodes are used in photomultiplier tubes to emit an electron in response to each impinging photon or a plurality of secondary electrons for each impinging primary electron. The primary electrons can be photoelectrons from a photocathode or secondary electrons from another dynode. The problem that has been encountered in the construction of phototubes has been to efficiently collect electrons emitted from one stage of the electron multiplier by another stage. In particular, the problem has been to maximize the collection of electrons at the input stage of the electron multiplier, i.e., photoelectrons from the photocathode to the first or primary dynode of an electron multiplier. An increase in the collection efficiency of electrons at the input stage increases the signal-to-noise ratio of the photomultiplier tube.
U.S. Pat. No. 4,112,325 to R. D. Faulkner, issued Sept. 5, 1978, and entitled, "An Electron Discharge Tube Having a Cup-Shaped Secondary Electron Emissive Electrode", describes a primary dynode having a relatively large area which provides a high collection efficiency for electrons emitted from the photocathode and incident thereon.
The collection efficiency of the above-described primary dynode may be improved by the addition of a focusing electrode disposed between the photocathode and the primary dynode for focusing photoelectrons emitted from the photocathode onto the primary dynode. Such a focusing electrode is described in the copending U.S. Patent Application, Ser. No. 65,842 by R. D. Faulkner et al., filed on Aug. 13, 1979, and assigned to the same assignee as the present patent application. The focusing electrode is also disclosed to focus secondary electrons emitted from the primary dynode onto the secondary dynode.
In many photomultiplier tube applications such as scintilation counting, for example, it is required that the output of the photomultiplier be linear with light input. Since the light energy of scintillations is directly proportional to the gamma ray energy over a certain range, an electrical pulse obtained from a photomultiplier tube is a direct measure of the gamma ray energy. Consequently, an important requirement of photomultiplier tubes used in scintillation counting is the ability to discriminate between pulses of various height. The parameter indicating the ability of a tube to perform this discrimination is called pulse-height resolution. The pulse-height resolution of a photomultiplier tube having a primary dynode such as that described in the Faulkner patent and in the copending Faulkner et al. application may be improved by increasing the secondary emission gain from the active area, i.e., the secondary emissive portion, of the primary dynode.
The secondary emission gain from a dynode is a direct function of the energy of the primary electrons incident on the dynode and also the composition and uniformity of the secondary emissive material. For example, oxidized beryllium copper dynodes typically have a secondary emission gain in the range of about 3 to 5 for a typical primary electron voltage range of about 100 to 200 volts. Other materials such as cesium-antimony on nickel dynode substrates are somewhat higher in secondary emission gain, ranging for example, from about 5 to 7 for primary electrons within the same 100 to 200 volt range. A photomultiplier tube having such a cesium-antimony dynode structure is described in U.S. Pat. No. 2,574,356 to Sommer, issued on Nov. 6, 1951, and entitled, "Process of Making Photoelectric Cathodes".
Significantly higher secondary emission gains in the range of 6 to 11 over the primary electron voltage range of 100 to 200 volts can be achieved using potassium, cesium, and antimony materials on a nickel dynode substrate. Such a structure is described in U.S. Pat. No. 3,753,023 to Sommer, issued on Aug. 14, 1973, and entitled, "Electron Emissive Device Incorporating a Secondary Emitting Material of Antimony Activated with Potassium and Cesium."
Since in most photomultiplier tubes, the number of stages usually range from 5 to 14, it follows that a small increase in secondary emission gain in the first stage will provide a large increase in the number of secondary electrons which reach the anode. Therefore, for similarly constructed photomultiplier tubes having an identical number of stages, the pulse-height resolution of a tube having a primary dynode with a relatively high secondary emission gain such as that provided by the alkali antimonides described in the above-mentioned Sommer's patents will be much greater than the pulse-height resolution of a tube using, e.g., an oxidized beryllium-copper dynode such as that described in the Faulkner patent and also in Faulkner et al. patent application cited above.
Where high secondary emission gain is desirable, potassium, cesium and antimony are often disposed on a nickel substrate to form a dynode. It is well known in the art, however, that problems are often encountered in evaporating antimony on nickel dynodes. Nickel has a tendency to alloy with antimony resulting in low secondary emission from dynodes formed from a nickel substrate and having an antimony layer activated by one or more alkali materials deposited thereon. Attempts to prevent this alloying, such as those described by C. M. Tomasetti et al. in U.S. Pat. No. 4,160,185 issued July 3, 1979, entitled, "Red Sensitivity Photocathode Having an Aluminum Oxide Barrier Layer", have been quite successful, however, the additional processing steps to prevent alloying of the nickel with the antimony increase the cost of the tube, and are, therefore, to be avoided if at all possible.
Uniformity of the secondary emission gain is also desirable in order to insure that the output of the photomultiplier tube remains linear regardless of where, on the primary dynode, the secondary electrons originate. For example, in the Sommer's patent (U.S. Pat. No. 2,574,356) the dynodes are disclosed to be of louvered, or "venetian blind", construction. Because of the angle of the louvers and the shielding provided by adjacent louvers, it is not possible to provide a sufficiently uniform layer of evaporated antimony on all parts of each louver. Accordingly, the secondary emission gain of the venetian blind dynode will be non-uniform.
Photomultiplier tubes have been produced by other manufacturers having oxidized beryllium-copper venetian blind dynodes and alkali antimonide photocathodes. In several of these tubes a single antimony evaporator is disposed between the faceplate and the dynodes, at about the center line of the tube. This evaporator location is selected in an attempt to provide a uniform antimony film on the faceplate for the subsequent formation thereon of a uniform photocathode. Of course, some antimony is also incidentally and unintentionally evaporated on the exposed surfaces of the first dynode and some antimony may also pass between the louvers in the first dynode to portions of the second dynode. Because of the shielding provided by the louvers, some portions of the dynodes will not receive any antimony and the resulting secondary emission surface formed on the dynodes will be non-uniform. The secondary emission gain variation from such a dynode surface will be so grossly non-uiform (i.e., highly position dependent) as to suggest the need for preventing the evaporation of antimony on oxidized beryllium-copper dynodes.